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A282135 Expansion of (Sum_{k = p*q, p prime, q prime} x^k)^3. 1
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 3, 0, 3, 3, 4, 6, 6, 3, 9, 9, 12, 12, 6, 10, 15, 18, 16, 12, 12, 21, 27, 27, 21, 18, 24, 30, 36, 36, 25, 27, 36, 49, 48, 36, 36, 51, 51, 54, 57, 66, 63, 42, 57, 75, 72, 66, 51, 69, 78, 79, 90, 102, 73, 75, 84, 117, 126, 84, 75, 105, 123, 121, 114, 120, 124, 102, 117, 156, 156, 129 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,14

COMMENTS

Number of ways to write n as an ordered sum of three semiprimes (A001358).

Conjecture: a(n) > 0 for n > 15.

LINKS

Table of n, a(n) for n=1..83.

Ilya Gutkovskiy, Extended graphical example

Eric Weisstein's World of Mathematics, Semiprime

FORMULA

G.f.: (Sum_{k = p*q, p prime, q prime} x^k)^3.

EXAMPLE

a(14) = 3 because we have [6, 4, 4], [4, 6, 4] and [4, 4, 6].

MATHEMATICA

nmax = 83; Rest[CoefficientList[Series[Sum[Floor[PrimeOmega[k]/2] Floor[2/PrimeOmega[k]] x^k, {k, 2, nmax}]^3, {x, 0, nmax}], x]]

CROSSREFS

Cf. A001358, A098238, A199331.

Sequence in context: A167223 A261922 A078907 * A278923 A210485 A111815

Adjacent sequences:  A282132 A282133 A282134 * A282136 A282137 A282138

KEYWORD

nonn

AUTHOR

Ilya Gutkovskiy, Feb 06 2017

STATUS

approved

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Last modified February 25 12:57 EST 2018. Contains 299654 sequences. (Running on oeis4.)